Powder precipitation of cellulose esters of fatty acids of 3-4 carbon atoms



Feb. 18, 1958 F. M. VOLBERG ETAL 2,

r I POWDER PRECIPITATION 0F CELLULOSE ESTERS 0F FATTY ACIDS OF 3-4 CARBON ATOMS Filed Feb. 4. 1955 2 Sheets-Sheet 1 [PR5 C/P/ TAN T olberg Melvin DMaz-iin IN V EN TORS' 1958 F. M. VOLBERG ETAL 2,824,098

POWDER PRECIPITATION OF CELLULOSE ESTERS 0F FATTY ACIDS OF 3-4 CARBON moms Filed Feb. 4, 1955 2 Sheets-Sheet 2 Pi ,2 l7 PRECIPITANT Lill HI Me [Vin D.Marfin/ V EN TORS 5 B Y t rm POWDER PRECIPITATION OF CELLULOSE ESTERS F FATTY ACllDS OF 3-4 CARBON ATOMS Frank M. Volberg and Melvin D. Martin, Kingsport,

Tenn., assignors to Eastman Kodak Company, Rochester, N. Y., a corporation of New Jersey Application February 4, 1955, Serial No. 486,186

7 Claims. (Cl. 260-230) This invention relates to the precipitation in powder form of cellulose esters containing more than of fatty acid groups of 3-4 carbon atoms in which the precipitating conditions are so adjusted that a product is obtained which can be readily washed free of acid and dried without the formation of fines.

In the manufacture of cellulose acetate, precipitates in the form of fiake may be obtained using a dilute aqueous acetic acid precipitant which products have an open form whereby the particles may be readily washed free of acid and dried. In the case of the esters containing appreciable contents of propionyl or butyryl it has been found to be desirable to add glacial acetic acid to the spent esterification mass before precipitation so as to obtain a soft particle from which acid may be readily washed, as disclosed in U. S. 2,339,631 of Fletcher and Crane. Without the addition of the acetic acid a hard impervious particle resistant to the removal of acid therefrom by washing is obtained. The precipitates obtained by prior procedures leave something to be desired in bulking properties, do not flow readily and do not lend themselves readily to proceessing into plastics. Other precipitation methods for cellulose esters have been known but the products obtained have often lacked in one or more properties such as case of washing, ease of centrifuging to a high percent solid content or drying without disintegrating or breaking up into smaller particles or fines.

One object of our invention is to prepare cellulose esters in powder form which are characterized by ease of washing acids therefrom, ease of centrifuging to a high solids percentage and of drying without breaking up of the mass into fines.

Another object of our invention is to provide a method for precipitating cellulose esters particularly of high propionyl or butyryl content requiring a much less acetic acid dilution than has been considered necessary heretofore in good precipitation operations. A further object of our invention is to provide a continuous precipitation method for preparing cellulose esters in powder form. Other objects of our invention will appear herein.

Our invention relates to the separation of cellulose esters of fatty acids of 2-4 carbon atoms in which the content of fatty acid radicals of 3-4 carbon atoms is more than 15% from their solutions in fatty acids as result from their preparation, which esters are obtained in the form of a powder susceptible to the action of solvents, washing liquids and the like. The ester precipitated in accordance with our invention may be substantially fully esterified or it may be an ester which had been subjected to some hydrolysis. In its broadest aspects our invention comprises starting with a spent esterification dope in which the cellulose ester is in solution, which dope has at least of acetic acid based on the total fatty acid present therein. In the case of those esters having a high acetyl content the content of acetic acid in the esterification bath will probably already be 2,824,093 Patented Feb. 18, 1958 sufficient whereas with those esters in which the propionyl or butyryl content is at least 25% it may be necessary to add acetic acid to the mass to impart the desired acetic acid content thereto and to contribute to the flowability thereof. If the dope results from a hydrolysis step there may be some acetic acid diluent already present therein. However, for a cellulose ester solution in which no hydrolysis has been employed, it may be desirable to add acetic acid. If desired, mixed acid may also be added to give still further dilution.

Initially the dope for the precipitation is preferably subjected to anincrease in water content therein to bring the composition of the mass near to but short of its break point. This may be accomplished either before introducing into a continuous apparatus or in one of a series of chambers. The break point of the cellulose ester may be described as that point in the progressive dilution of the acid dope with precipitant (water or aqueous acid) where the first significant portion of cellulose ester comes out of solution. At this point a small increment of precipitant results in a major decrease in viscosity of the system and causes a considerable portion of the cellulose ester to come out of solution. At no other stage during the progressive aqueous dilution of the dope is there such a sharp decrease in viscosity of the system.

This point is a critical phase in our powder precipitation process and the chamber in which this major decrease in viscosity occurs and the cellulose ester precipitates in substantial amount is termed the initial precipitation compartment. We have found that a better product may be obtained by effecting reduction of the acid concentration of the dope in discrete steps, as is accomplished in the successive compartments of our continuous precipitation process, instead of in an infinite number of tiny stages, as in batchwise precipitation. This principle is particularly important in the range of acid concentrations just below the break point. In the initial precipitation compartment the entire volume of the liquid therein, except that immediately adjacent the acid and dope inlets thereof, is maintained at an acid co11 centration selected from the range of 3l2% below the break point of the cellulose ester dope which is introduced therein. It will be understood that, where acid concentration is referred to above and hereinafter, the percent acid "given will be the titrated percent acid, reported as percent acetic acid.

It is often desirable in the carrying out of a precipitation process in accordance with our invention that the break point concentration of the mass be approached in a preliminary operation. This may be done either by independently decreasing the acid concentration of the mass to a point where it is only 2 or 3% above the break point concentration, such as by adding aqueous acid, or by passing the dope through one or more compartments in which the acid concentration is reduced until slightly above the break point, prior to introducing the dope into the initial precipitation compartment. Upon introduction of the dope continuously into the initial precipitation compartment there is also continuously introduced aqueous fatty acid in sufficient amount to main tain a selected acid concentration within the range of 3-12% below the break point of the dope, the dope and the acid being introduced slowly enough that only immediately adjacent the inlets therefor is the selected acid concentration not present. The mass in this compartment is subjected to vigorous agitation. The stirrer therein is ordinarily run at a speed within the range of 200-600 R. P. M. except where larger chambers are used in which case a lesser R. P. M. will accomplish vigorous agitation.

As the dope and the acid are continuously introduced into the initial precipitation compartment the slurry which they replace continuously moves on to the next compartment in which an acid concentration is maintained lower, but not more than 12% 'lower,'than the selected acid concentration in the initial precipitation compartment except for the areas immediately adjacent the acid and dope inlets thereof. The compartment following the initial precipitation compartment will be termed herein the immediately following chamber. The contents of this immediately following compartment are vigorously agitated while dilute aqueous fatty acid is continuously introduced therein in such'amount that the slurry being continuously introduced *is reduced to the acid concentration being maintained in that compartment. In theinitial precipitation compartment the aqueous acid introduced is preferably of 30-40% concentration so as to provide extra liquid and to facilitate agita tion in this initial compartment.

The dope is continuously moved on through one or more additional compartments in which more aqueous conditions prevail so as to precipitate any cellulose ester which might still be in solution, and to harden the precipitate already formed. This may utilize several compartments in which the concentration of the mass is progressively reduced until down to a concentration of 35-40% or even lower such as down to 20% or less. The criterion of the final acid strength is that the'resulting slurry is susceptible to easy separation of the solid particles therefrom or in other words that the cellulose ester particles have been sufliciently hardened. The precipitate obtained is separated from the slurry in which it is found such as by filtering or centrifuging and is washed and dried. The product obtained has excellent physical properties.

In carrying out the precipitation process in accordance with our invention there may be employed one or several compartments prior to the initial precipitation and the concentration of the mass can be gradually reduced to near the break point concentration continuously or it can be reduced in but one receptacle immediately prior to its introduction into the initial precipitation compartment.

In the apparatus to be employed for continuous precipitation in accordance with our invention any type of agitator may be employed which vigorously agitates the liquid during the precipitation process. The blades may be of the propeller type or of some other configuration, preferably with baffles present in the chambers in which they are used to aid in the'mixing of the liquid undergoing the precipitation process. The apparatus employed may be a horizontal series of mixing chambers such as shown in Fig. l. of the accompanying drawings. 'It may be a mixing apparatus such as shown in Fig. 2 employing a series of connected chambers or -it-may be a continuous turbo type mixer of the-type shown in Perrys Chemical Engineers Handbook, 3rd edition, 1950, page 1211. In the accompanying drawings Figures 1 and 2 illustrate types of continuous apparatus which may be employed in a precipitation operation in accordance with our invention. Fig. 3 illustrates an agitated vessel in which a batchwise process may be carried out, over which operation the present invention is an improvement.

Referring to Fig. l, a dope of a hydrolyzed mixed cellulose ester of fatty acids of 2-4 carbon atoms in which ester fatty acid radicals of 3-4 carbon atoms constitute more than 15%, enters the apparatus through conduit 1 and is agitated in compartment A by stirrer 2 on a shaft 3 driven at a speed of'200600 R. P. 'M., the driving mechanism not 'being shown. The dope from compartment A continuously passes through the apparatus into successively agitated compartments'B, C, D, E, F, G and H through the successive annular spaces, 4. Agitator blades are provided in each compartment. Bafdes in the form of rods, 5, are provided :to improve the agitation. Inlets 6 are provided in each compartment 4 for the introduction of aqueous acid thereto. These inlets are supplied from a suitable diluent header and each is provided with a valve and rotometer or some other type of control device as may be convenient to allow individual control of the dilution to each compartment separately. Sampling cocks such as 7 and a drain 8 are also provided. The slurry of cellulose ester precipitate which results is removed from the apparatus through conduit 9.

The apparatus illustrated in Fig. 1 is adapted to run full and the liquid is continuously moved along by the pressure from the acid and dope which is introduced in the operation of the precipitation method. The apparatus may contain a greater or lesser number of compartments than illustrated but it is desirable that at least 3 and preferably at least 5 compartments be used in precipitating cellulose esters by our continuous precipitating method.

'Fig. 2 illustrates another type of apparatus suitable for the practice of our invention. The apparatus 'here consists of a series of separate, interconnected, agitating vessels in which the starting dope enters the first vessel L through the conduit 11 where it is agitated by the stirrer blades 12, in conjunction with baffles 13 and 14, suitably energized by means not shown. Aqueous acid is added to vessel L through conduit 15, the flow rate of this acid being controlled by valve '16 and rotometer 17. The uniformly mixed contentsof vessel L are continuously passing through conduit 18 to vessel M as further dope is led into vessel L through conduit 11. In the illustration the flow is induced by gravity by a suitable vertical spacing of the two vessels. If desired, a pump may be provided to-move the liquid between these vessels. The succeeding vessels M, N, O and P have provision for agitation and for the introduction of aqueous acid and are provided with conduits so that the mass continuously passes from one vessel to the next.

The product in vessel P is a slurry of the cellulose ester in divided form in a mixture of the acid and water. The slurry is then led to succeeding draining, washing and drying operations by processes well known to those skilled in the cellulose ester art.

Fig. 3 shows a single vessel for the batehwise precipitation of cellulose esters in which the dope is led to the vessel through conduit 21 where it is agitated by stirrer 22 against baffles 23 and 24 which stirrer is energized by means not shown. The conduit 25 with valve 2 6and rotometer 27 is provide'dforthe addition-of acid which may be desirable for diluting-the dope before the precipitation occurs. The conduit 28 provided with valve 29 and rotometer 30 is provided for the addition of'more dilute aqueous acid for the precipitation operation proper. The initial charge to the vessel must be small enough to assure that that vessel can contain the entire quantity of diluting acid and precipitating acid which will be added after the precipitation has occurred. The slurry obtained may be drained away through valve Sland further processed.

The following examples illustrate our invention; Example 1 illustrating the preparation of a cellulose acetate butyrateesterification mass, Examples 2 and 7 illustrating 'batchwise procedures for precipitating cellulose esters and our precipitation method being illustrated by Examples 3, 4, 5 and 6.

Example 1 Cellulose acetate butyrate was prepared by esterifying 600 lbs. of refined cotton linters with an esterification mixtnreof 600 lbs. of acetic acid, 600 lbs. of butyric acid and 2400 lbs. of butyric anhydride using sulfuric acid catalyst. Upon completion of the esterification 1200 lbs. of aqueous aceticacid of 50% strength was added and the ester was hydrolyzed for .a short time. The resulting mass consisted of 1'200lbs. of cellulose acetate butyrate' of 38% butyryl content and 12.5% acetyl content, 2726 ass-apes a: al lbs. of butyric acid, 974 lbs. of acetic acid and 500 lbs. of water together with a small amount of sulfuric acid.

Example 2 period using vigorous agitation; enough water being added to lower the acid concentration to 38%. A very fine hard powder was obtained, which powder was readily washable. Example 3 A cellulose acetate butyrate dope was prepared following the procedure described in Example 1 and was diluted with 5780 lbs. of 75% acetic acid which gave a composition of 1200 lbs. cellulose acetate butyrate, 2726 lbs. of butyric acid, 5309 lbs. of acetic acid and 1945 lbs. of water. This solution was passed through a continuous tubular precipitator containing 8 compartments as shown in Fig. 1 while 35 aqueous fatty acid was being continuously added in the first compartment to maintain the acid concentration therein at 65%, the break point of the dope being ca. 59% acid concentration. The introduction of dope and acid in the first compartment pushed a corresponding quantity of the mass into the next compartment where 35% aqueous fatty acid was being introduced and the mass was vigorously agitated, to maintain an acid concentration of the contents of this compartment of 54%, this chamber B, the initial precipitation compartment. Whereas the viscosity of the dope in compartment A was within the range of 1000 cps. that in compartment B having an acid concentration of 54% was about 50 cps. and in this compartment B a considerable portion of the cellulose ester came out of solution. In compartment B only in the areas immediately adjacent the acid intake and the aperture through which the dope was slowly introduced into compartment B was there any deviation from the 54% acid concentration. With the continuous introduction of dope and acid into compartment B the slurry which forms in that compartment is continuously pushed into compartment C the contents of which are maintained at an acid concentration of approximately 48% by introducing therein 8% aqueous fatty acid accompanied by vigorous agitation. In compartment C the only deviation from the 48% acid concentration is in the areas immediately adjacent its inlets. The slurry formed in compartments B and C is continuously pushed along from compartment to compartment the contents of which are maintained at progressively lower acid concentrations respectively until a 35% acid concentration is reached in compartment H.

The slurry in compartment H continuously exits therefrom. Upon draining, washing and drying a very fine, hard, uniform powder is obtained, the product having good stability properties. The powder thus obtained Washes readily to a very low acid concentration and centrifuges to a very high solids content.

Example 4 A cellulose acetate butyrate dope as prepared in Example 1 was diluted with 2360 lbs. of 66% acid giving a composition of 1200 lbs. of cellulose acetate butyrate, 2726 lbs. of butyric acid, 2532 lbs. of acetic acid and 1302 lbs. of Water. This mass was continuously pumped into a 5 stage precipitator as illustrated in Fig. 2. In the first vessel L acid was added to maintain the acid concentration therein at 66%, the break point being ca. 60%. In the second vessel M 35 acid was continuously added soas to maintain the acid concentration at 57% as liquid continuously passes from vessel L to M. This vessel M is the initial precipitation compartment. In the third vessel N 10% acid is continuously introduced to maintain the acid concentration at 52% therein as the contents of vessel M continuously enter, thereby conditioning the precipitated particles suspended in the liquid therein. The slurry formed continuously passes on into vessels 0 and then P and 10% strength acid is continuously added to each so that its acid concentration is brought progressively to a final concentration of 30%. The slurry continually exits from vessel 1?. Upon draining, washing and drying a fine, hard, uniform free-flowing powder is obtained in ideal form for converting into plastics.

Example 5 A. dope resulting from the preparation of a cellulose acetate butyrate containing 970 lbs. of cellulose acetate butyrate having a butyryl content of 17.5% and an acetyl content of 30%, 2500 lbs. of butyric acid, 2700 lbs. of acetic acid and 400 lbs. of water was diluted with 1440 lbs. of 30% acetic acid. The mass was precipitated by slowly passing in a continuous manner through a 5-stage continuous precipitator as shown in Fig. 2.

The dope was stirred with aqueous acid of 35% rength in vessel L to reduce the acid concentration of the dope to 53%, the break point of this dope being ca.

50% acid concentration. As dope is introduced into vessel L liquid therefrom is continuously moved along to vessel M where 35 acid is also continuously intro duced into the vigorously agitated contents thereof to maintain an acid concentration therein of 45%, which gives a mass of greatly lower viscosity than in vessel L and the precipitation from solution in the form of fine particles of a large portion of the cellulose ester therein. As liquid enters vessel M, liquid containing suspended particles therefrom continuously passes into vessel N to which 10% acid continuously passes to maintain an acid concentration of 41% thereby conditioning the solid particles therein. The slurry in N continuously passes on to vessel 0 where an acid concentration of 35% is maintained by aqueous acid addition and from O to vessel P Where an acid concentration of 30% is maintained by adding aqueous acid, the contents of all of the vessels being vigorously agitated.

The slurry continuously exits from vessel P and upon draining, washing and drying, a uniform, easily washable product is obtained.

Example 6 A spent esterification mass containing 1200 lbs. of cellulose propionate having a propionyl content of 54%, which dope contained also 1200 lbs. of acetic acid, 600 lbs. of water and 3000 lbs. of propionic acid was diluted with 5780 lbs. of 75% strength acetic acid. The mass was precipitated by passing in a continuous manner through the 5-stage precipitator as shown in Fig. 2. The break point acid concentration and the concentrations maintained in the various vessels in the operation being as follows:

Break point Ca. 65% acid concentration.

Vessel:

upon draining, washing and drying the slurry which con tinuously exits from the vessel P, a cellulose propionate powder of good characteristics resulted.

Example 7 A cellulose acetate butyrate was prepared as described in Example 1. This mass was diluted with 5780 lbs. of 75% acetic acid to give a composition of 1200 lbs. of

cellulose acetate butyrate, 2726 lbs. of butyric acid, 5309 lbs; of acetic acid and 1945 lbs. of water. The mass was then introduced into a mixer as shown in Fig. 3 and was precipitated by adding weak aqueous acid slowly over a period of 1 hour to reduce the concentraction to 35% using vigorous agitation throughout. This procedure gave a large, very hard and impervious powder which was diflicult to Wash free of acid. The final product had poor color and heat stability.

In the examples given it is desirable that the acid concentrations in the two compartments regarded as critical, namely, the first compartment containing precipitated ester and the first conditioning compartment, be maintained by the addition of aqueous acid of ca. 35-40% strengti. In the less critical further conditioning compartments acids of lower concentration may be added to maintain the selected acid concentration. The aqueous acid added to these compartments may be either aqueous acetic acid or other aqueous acids such as result from previous precipitations of propionic or butyric acid esters of cellulose. Our invention is of value in precipitating any of the following cellulose esters from the acid baths in which they are dissolved as a result of their preparation, Whether with or without subsequent hydrolysis: cellulose acetate propionate, cellulose acetate butyrate, cellulose acetate isobutyrate, cellulose butyrate, cellulose propionate, cellulose isobutyrate, cellulose propionate isobutyrate, and cellulose acetate propionatc butyrate, having more than of fatty acid radicals.

It is desirable in precipitating cellulose esters in accordance with our invention that the acid solution in which those esters are dissolved have therein at least 25% of acetic acid based on the total organic acid therein. Also the solution may have water therein such as up to a proportion of -30% or even up to near but short of the break point ofthe solution, this proportion being based on the total of the fatty acid and water present in the mass.

In carrying out precipitations in accordance with our invention the temperature employed has an effect on the size of the particles obtained. The use of lower temperatures gives smaller particles. Therefore, it is preferred that the precipitation process be operated at or only slightly above room temperature to obtain a product of good particle size. However, the process can be successfully operated at higher temperatures such as 100 F. or higher particularly in the case of the cellulose esters in which the proportion of propionyl or butyryl is in the lower part of the range given. Another factor influencing the size of the particles is the amount of agitation which is employed. If fine particle size is desired, it is desirable to use a higher degree of agitation, other things being equal, than for obtaining coarser particles.

in the carrying out of precipitation in accordance With our invention, it is important to maintain the initial precipitation compartment at an acid concentration of 3-12% below the break point. If the concentration in this vessel is too low such as lower than 12%, the tendency is to form a flake precipitate rather than a powder. The break point concentration varies to some extent depending upon the individual ester used and on the amount of acetic acid dilution, if any, employed. Ordinarily the dope break points of esters precipitated in accordance with our invention may vary from a concentration of about 50% to about 70% acid. in general it may be said that the higher the proportion of higher acy-l on the ester the higher the break point whereas the greater'the proportion of acetic acid in the dope the lower the break point.

We claim:

l. A method of precipitating in a continuous manner cellulose esters of fatty acids of 2-4 carbon atoms having a content of fatty acid radicals of 3-4 carbon atoms of morethan 15 from their solutions in mixed fatty acids in which acetic acid constitutes at least of the acid, which comprises continuously introducing a solution of the cellulose ester in mixed fatty acid into one of a series of chambers wherein the entire volume therein is highly agitated and is maintained by the continuous addition of aqueous acid at an acid concentration selected from the range of 3-l2% below the break point of the cellulose ester solution, except in the areas immediately adjacent the intakes in the chamber for the solution and for the aqueous acid, to maintain the selected acid concentration, whereby a slurry is formed, continuously passing the soformed slurry to the next of the series of chambers in which the entire volume therein is highly agitated and is maintained by the continuous addition of aqueous acid at an acid concentration below, but not more than 12% below, the acid concentration value of the preceding chamber, except immediately adjacent the intakes thereof, and further continuously passing the slurry along the series of chambers in which the acid concentration is further diminished until the slurry can be withdrawn from the last chamber of the series and the solid particles therein be separated from the liquid in which those particles are suspended so as to obtain the cellulose ester in the form of a mass of uniform, readily washable powder.

2. A method of precipitating in a continuous manner cellulose acetate butyrate having a butyryl content of more than 15% from its solution in mixed fatty acids in which acetic acid constitutes at least 25% of the acid, which comprises continuously introducing the solution into one of a series of chambers wherein the entire volume therein is highly agitated and is maintained by the continuous addition of aqueous acid at an acid concentration selected from the rang of 3-12% below the break point of the cellulose ester solution, except in the areas immediately adjacent the intakes in the chamber for the solution and for the aqueuous acid to maintain the se lected acid concentration, whereby a slurry is formed, continuously passing the so-formed slurry to the next of the series of chambers in which the entire volume therein is highly agitated and is maintained by the continuous addition of aqueous acid at an acid concentration below, but not more than 12% below, the acid concentration value of the preceding chamber, except immediately adjacent the intakes thereof, and further continuously passing the slurry along the series of chambers in which the acid concentration is further diminished until the slurry can be Withdrawn from the last chamber of the series and the solid particles therein be separated from the liquid in which these particles are suspended so as to obtain the cellulose ester in the form of a mass of uniform, readily washable powder.

3. A method of precipitating in a continuous manner cellulose acetate propionate having a propionyl content of more than 15% from its soltuion in mixed fatty acids, in which acetic acid constitutes at least 25% of the acid, which comprises continuously introducing the solution into one of a series of chambers wherein the entire volume therein is highly agitated and is maintained by the continuous addition of aqueous acid at an acid concentration selected from the range of 3-i2% below the break point of the cellulose ester solution, except in the areas immediately adjacent the intakes in the chamber for the solution and for the aqueous-acid to maintain the selected acid concentration, whereby a slurry is formed, continuously passing the so-formed slurry to the next of the series of chambers in which the entire volume therein is highly agitated and is maintained by the continuous addition of aqueous acid at an acid concentration below, but not more than -12% below, the acidconcentration value of the preceding chamber, except immediately adjacent the intakes thereof, and further continuously passing the slurry along the series of chambers in which the acid concentration is further diminished until the slurry can be withdrawn from the last chamber of the series and the solid particles therein be separated from the liquid in which those particles are suspended so as to obtain the cellulose ester in the form of a mass of uniform, readily washable powder.

4. A method of precipitating in a continuous manner a cellulose ester of fatty acids of 2-4 carbon atoms and having a content of fatty acid radicals of 34 carbon atoms of more than 15% from their solutions in mixed fatty acids in which acetic acid constitutes at least 25% of the acid, which comprises introducing the solution into a series of chambers in the first of which the solution is adjusted to a composition near to but short of the break point of the solution by adding aqueous fatty acid thereto, continuously passing solution therefrom into the next of the series of chambers wherein the entire volume therein is highly agitated and is maintained by the continuous addition of aqueous acid at an acid concentration selected from the range of 3-12% below the break point of the solution, except in the areas immediately adjacent the intakes of the chamber for the solution and for the aqueous acid to maintain the selected acid concentration, whereby a slurry is formed, continuously passing the so-formed slurry to the next of the series of chambers wherein the entire volume therein is highly agitated and is maintained by the continuous addition of aqueous acid at an acid concentration below, but not more than 12% below, the acid concentration value of the preceding chamber, except immediately adjacent the intakes thereof, and further continuously passing the slurry along the series of chambers in which the acid concentration is further diminished until the slurry thus obtained can be withdrawn from the last chamber of the series and the solid particles therein be separated from the liquid portion in which those particles are suspended so as to obtain a cellulose ester mass in the form of a uniform, readily washable powder.

5. A method of precipitating in a continuous manner cellulose acetate butyrate having a butyryl content of at least 25 as prepared in an esterification bath in which aqueous acid is added and the ester is hydrolyzed, followed by adjusting of the acid composition so that acetic acid constitutes at least 50% of the total acid present, which comprises thoroughly mixing aqueous fatty acid into the cellulose ester solution so-obtained to bring its acid concentration near to but short of its break point, continuously introducing the solution into one of a series of chambers wherein the entire volume therein is highly agitated and is maintained by the continuous addition of aqueous acid at an acid concentration selected from the range of 312% below the break point of the solution, except in the areas immediately adjacent the intakes of the chamber for the solution and for the aqueous acid to maintain the selected acid concentration, whereby a slurry is formed, continuously passing the so-formed slurry to the next of a series of chambers in which the entire volume therein is highly agitated and is maintained by the continuous addition of aqueous acid at an acid concentration below, but not more than 12% be-- low, the acid concentration value of the preceding chamber, except immediately adjacent the intakes thereof, and further continuously passing the slurry along the series of chambers in which the acid concentration is further diminished until the slurry can be withdrawn from the last chamber of the series and the solid particles therein be separated from the liquid in which those particles are suspended, so as to obtain the cellulose ester in the form of a mass of uniform, readily washable powder.

6. A method of continuously precipitating a high butyryl cellulose acetate butyrate in powder form which comprises esterifying cellulose in an esterification mass in which butyric anhydride predominates, adding aqueous acetic acid to the mass and allowing the cellulose ester to hydrolyze for a short time, diluting the mass with aqueous acetic acid and continuously introducing into one of a series of chambers wherein the entire volume therein is highly agitated and is maintained, by the continuous addition of aqueous acid, at an acid concentration selected from the range of 312% below the break point of the solution except in the areas immediately adjacent the intakes in the chamber for the solution and for the aqueous acid to maintain the selected acid concentration, whereby a slurry is formed, continuously passing the soformed slurry to the next of the series of chambers in which the entire volume therein is highly agitated and is maintained by the addition of aqueous acid, at an acid concentration below, but not more than 12% below, the acid concentration value of the preceding chamber except in the areas immediately adjacent the intakes thereof and further continuously passing the slurry along the series of chambers in which the acid concentration is further diminished until a concentration of 35% is reached, withdrawing the slurry thus obtained from the last chamber of the series and separating the solid particles suspended therein from the liquid portion of the mass so as to form a cellulose acetate butyrate powder having readily washable characteristics.

7. A method of precipitating in a continuous manner cellulose acetate isobutyrate from its solution in mixed fatty acids, in which acetic acid constitutes at least 25% of the acid, which comprises continuously introducing the solution into one of a series of chambers wherein the entire volume therein is highly agitated and is maintained, by the continuous addition of aqueous acid, at an acid concentration 3-l2% below the break point of the cellulose ester solution, except in the area immediately adjacent the intakes in the chamber for the solution and for the aqueous acid to maintain the selected acid concentration, whereby a slurry is formed, continuously passing the so-formed slurry to the next of the series of chambers in which the entire volume therein is highly agitated and is maintained by the continuous addition of aqueous acid at an acid concentration below, but not more than 12% below, the acid concentration value of the preceding chamber, except immediately adjacent the intakes thereof, and further continuously passing the slurry along the series of chambers in which the acid concentration is further diminished until the slurry can be withdrawn from the last chamber of the series and the solid particles therein be separated from the liquid in which those particles are suspended so as to obtain the cellulose ester in the form of a mass of uniform readily washable powder.

References Cited in the file of this patent UNITED STATES PATENTS 2,036,397 Clarke et a1. Apr. 7, 1936 2,063,322 Malm et al. Dec. 8, 1936 2,208,637 Jones et al. July 23, 1940 2,239,782 Haney Apr. 29, 1941 

1. A METHOD OF PRECIPITATING IN A CONTINUOUS MANNER CELLULOSE ESTERS OF FATTY ACIDS OF 2-4 CARBON ATOMS HAVING A CONTENT OF FATTY ACID RADICALS OF 3-4 CARBON ATOMS OF MORE THAN 15% FROM THEIR SOLUTIONS IN MIXED FATTY ACIDS IN WHICH ACETIC ACID CONSTITUENTS AT LEAST 25% OF THE ACID, WHICH COMPRISES CONTINUOUSLY INTRODUCING A SOLUTION OF THE CELLULOSE ESTER IN MIXED FATTY ACID INTO ONE OF A SERIES OF CHAMBERS EHWERIN THE ENTIRE VOLUME THEREIN IS HIGHLY AGITATED AND IS MAINTAINED BY THE CONTINUOUS ADDITION OF AQUEOUS ACID AT AN ACID CONCENTRATION SELECTED FROM THE RANGE OF 3-12% BELOW THE BREAK POINT OF THE CELLULOSE ESTER SOLUTION, EXCEPT IN THE AREAS IMMEDIATELY ADJACENT THE INTAKES IN THE CHAMBER FOR THE SOLUTION AND FOR THE AQUEOUS ACID, TO MAINTAIN THE SELECTED ACID CONCENTRATION, WHEREBY A SLURRY IS FORMED, CONTINUOUSLY PASSING THE SOFORMED SLURRY TO THE NEXT OF THE SERIES OF CHAMBERS IN WHICH THE ENTIRE VOLUME THEREIN IS HIGHLY AGITATED AND IS MAINTAINED BY THE CONTINUOUS ADDITION OF AQUEOUS ACID AT AN ACID CONCENTRATION BELOW, BUT NOT MORE THAN 12% BELOW, THE ACID CONCENTRATION VALUE OF THE PRECEDING CHAMBER, EXCEPT IMMEDIATELY ADJACENT THE INTAKES THEREOF, AND FURTHER CONTINUOUSLY PASSING THE SLURRY ALONG THE SERIES OF CHAMBERS IN WHICH THE ACID CONCENTRATION IS FURTHER DIMINISHED UNTIL THE SLURRY CAN BE WITHDRAWN FROM THE LAST CHAMBER OF THE SERIES AND THE SOLID PARTICLES THEREIN BE SEPARATED FROM THE LIQUID IN WHICH THOSE PARTICLES ARE SUSPENDED SO AS TO OBTAIN THE CELLULOSE ESTER IN THE FORM OF A MASS OF UNIFORM, READILY WASHABLE POWDER. 